1. Field of the Invention
The present invention relates to a visible radiation sensitive composition comprising a novel sensitizer which exhibits high sensitivity to light in the visible region.
2. Description of the Related Art
Conventionally, photosensitive compositions have been widely employed for various uses such as photoresists, engraving materials for litho graphic printing and relief printing, PS plates for offset printing, information recording materials and materials for relief image preparation.
Many of these compositions have sensitivity to ultraviolet light. The sensitivity, however, is generally from several dozens to several hundreds mj/cm.sup.2 and has disadvantages that high output is required of a light source and energy conversion efficiency for recording is low. On the other hand, there is a method for forming the image by direct depiction with a light source such as laser which has a high energy density. The method has advantages that the energy conversion efficiency is improved and image forming step is greatly simplified. In the method, visible laser is more favorable than ultraviolet laser as a scanning exposure light source for direct depiction because oscillation beams obtained are stable in view of life and intensity. Consequently, it has been desired to develop a visible radiation sensitive composition which is sensitive to scanning exposure by visible laser, particularly desired visible radiation sensitive composition is a composition which has a high sensitivity to Ar.sup.+ laser having stable oscillation beams in the visible region of 488 nm or 514.5 nm in wavelength.
However, photosensitive compositions which can satisfy the above demands have not yet been in practical use.
One of the primary reason is that there are few photosensitive resins which are sensitive to light in the region of long wavelengths.
The second reason is that a suitable photo-polymerization initiator system including a sensitizer has not yet been found.
Several proposals have conventionally been made for a photo-polymerization initiator system which is effective for visible radiation. The systems include, for example, a combination of hexaarylbisimidazol, and p-dialkylaminobenzylidene ketone or diaminoalkylchalcone (U.S. Pat. No. 3,652,275, Japanese Laid-Open Patent SHO 54-155292), a combination of camphorquinone and dye (Japanese Laid Open Patent SHO 48-084183), a combination of diphenyliodonium salt and Michler's ketone (GB. 020,297SA), a combination of S-triazine based compound and merocyanine dyestuff (Japanese Laid-Open Patent SHO 54-151024) and a combination of S-triazine based compound and thiopyrylium salt (Japanese Laid-Open Patent SHO 58-040302). Although the above initiator systems have initiation ability for light beams in the visible region, sensitivity is still unsatisfactory and compatibility with components of photopolymerizable compositions is also insufficient. A more practical photopolymerization initiator system having high sensitivity is desired.